This invention relates to a method for urine flow measurement wherein liquid in the form of a free-falling jet is collected in a collecting vessel, and the weight of the vessel as a vertically acting force is continuously sensed and utilized for yielding a measure of the flow.
The invention also relates to a flow meter for carrying out the method.
During examination of the function of the lower ureters of a human being, the measurement of the urine flow as a function of the time, is the objective examination method which offers the greatest potential possibilities for obtaining copious information. Based upon the urine flow and the hydrostatic pressure in the bladder of a person, the elastic properties of the urethra determining the urine flow can be obtained. It is a prerequisite, however, that this flow and pressure can be measured accurately. Known urine flow meters, however, yield serious errors in measurement.
In a usual method of flow measurement, a scale with a collecting vessel is used. The force, F, recorded by such a scale is given by the equation ##EQU1## where m.sub.o is the initial mass of the vessel, g is the acceleration of gravity, p is the density of the urine, t is the time, Q the volume flow for the urine, and v the corresponding flow rate. The integral term in equation 1 corresponds to the volume. The last term in the expression is the reaction force due to the kinetic energy of the urine jet.
In order to receive an output signal corresponding to the flow, the first derivative of the force signal is determined, whereby ##EQU2## The first term of the derivative, i.e. gpQ, is proportional to the desired flow. The remaining two terms are false signals occurring as a result of flow variations. The effect of these false terms can be illustrated by the recorded flow at an imagined pulse shaped uring flow. When the flow starts and ceases, due to said reaction force in principle infinitely large derivative terms are obtained which yield large overshoots in the measured valves. These overshoots can be filtered out, but the system then receives a very slow response.
Some of the problems with the kinetic energy of the urine jet can be overcome by collecting the urine in a funnel intended to take up the reaction force. The funnel, however, introduces a problem which potentially is still more serious, viz. a variable delay of the flow. This delay depends on where the urine jet meets the funnel. When a jet with constant flow is caused to oscillate over the funnel, this variable delay gives rise to a false variation in the flow which is recorded by the equipment.
In a known type of flow meter the flow is related to the effect which is required to accelerate the liquid to the circumferential speed of a rotating plate. The urine, however, here is collected by a funnel, and therefore the method and meter are afflicted with the errors in measurement involved therewith. When the urine jet is caused to directly meet the rotating plate, the reaction force of the jet affects the necessary effect for maintaining the rotation of the plate constant.